The invention relates to bore hole flow meters, and in particular, to a rotary bore hole flow meter which accurately detects and digitally interprets both the direction and the rate of fluid flow within the bore hole.
In the extraction of oil and other petroleum products from the earth, a bore hole is drilled down through multiple strata of earth. The bore hole is conventionally lined with a tubular casing which is selectively perforated to provide communication between oil producing strata and the bore hole. Fluids are conducted from the lower open end of the tubing, back up the bore hole to the surface. In a conventional well, it is desirable to be able to measure the rate of flow of the fluid and the direction of flow thereof by use of a tool which can be temporarily lowered into the bore hole to make such measurement.
In certain other types of petroleum production, a technique known as "water flooding" is employed, wherein water is pumped through a first bore hole to enter surrounding earth strata or formations and force oil contained in the strata into an adjoining producing well. That is, in petroleum production, fluids are forced both into subterranean formations and withdrawn from subterranean formations from various bore hole locations. Consequently, it is desirable to have a means for measuring, downhole, both the direction of flow as well as the rate of flow of fluids within the bore hole. It is preferred to have a simple and accurate flow meter which may be lowered into the bore hole at different depths and which will indicate both the direction and the rate of the flow of the fluids therein.
Prior art flow meter tools for boreholes have included devices for use in well bores with and without packers. Such flowmeters generally include a passageway open at inlet and outlet orifices to the exterior of the tool and a spinner section which measures the rate of fluid flow through the passageway. In a packer-type flowmeter, the horizontal cross section of the well bore, say seven inches in diameter, is packed off and the entire fluid flow is directed through the tool for fairly low rates of flow therein sampling the flow.
In a typical flowmeter spinner section, a spinner is rotated under the influence of the fluid flow at an angular velocity proportional to the velocity of the fluid flow, and the rotation is directed by a sensing system to provide an indication of the velocity of fluid flow. In the typical spinner assembly, particularly for low rates of fluid flow, it is difficult to physically mount the spinner for perfectly free rotation and to detect the true angular velocity of the spinner to provide accurate indications because of friction forces and other retarding forces in the assembly. Such retarding forces include spinner bearing resistance as well as sensor inertial and functional resistances.
Certain problems in prior art flowmeters are the results of the aforesaid resistance aspects of the tool. For example, it is desirable to detect very low fluid velocities in the range of ten to twenty feet per minute. Moreover, the use of packers is not always feasible or practical. Flow meters utilizing spinners or similar propeller structures having low performance efficiency may critically reduce the sensitivity of the tool. Flowmeter sensor units incorporating conventional magnetic pick-up units may also adversely affect sensitivity through start up torque and inertial resistance. Additionally, most prior art sensor units utilize a basic analog signal network which is sensitive to changes in temperature and subject to undiagnosed component error or failure; with low fluid velocities and/or related turbulence in the borehole, signal errors can be critical to proper logging operations and detection of fluid flow direction as well as flow rate.
It is equally important to detect and monitor fluid flow direction with equal sensitivity and accuracy. Many prior art units discriminate between analog signals to distinguish direction; but spinner efficiency, bearing resistance and component eror can render ineffective proper downhole monitoring of direction, as well as rate. In packerless designs, such factors as laminar flow conditions can alter flow reading accuracy, particularly between opposing flow directions. For example, a laminar flow condition will exist from a downward flow from the tool string to the spinner or related propeller element. The same condition will not exist in an upward flow. With the added sensitivity, fluctuations due to temperature and end loading resistance on certain prior art constructions, low or erratic fluid flow can remain undetected or recorded with any degree of accuracy.
It would be an advantage therefore, to overcome the aforesaid problems and disadvantages of prior art flowmeters. The flowmeter apparatus of the present invention is provided in a highly sensitive and reliable configuration which permits the detection and digital sensing of borehole fluid flow. A spinner structure is provided in an impeller configuration having helical vanes disposed therearound. The impeller is magnetically coupled to a digital sensing unit, incorporating a sealed optical chopper having virtually zero functional resistance as compared to magnetic flux sensors. In this manner, a flow meter is provided, having extremely low and accurate threshhold velocity detection capabilities.